1. Field of the Invention
The present invention relates to an image forming apparatus based on an electrophotographic method, which exposes a photosensitive member to a plurality of laser beams.
2. Description of the Related Art
It is conventionally known that in an image forming apparatus based on an electrophotographic method, such as a laser printer or a copying machine, a light beam scanning device that emits laser beams is generally used to form an electrostatic latent image on a photosensitive drum (photosensitive member).
The image forming apparatus based on the electrophotographic method uses a light beam scanning device. The light beam scanning device deflects a laser beam converted to a collimated laser beam by a collimator lens, using a polygon mirror, and the deflected laser beam is passed through an elongated f-θ lens to form an image on the photosensitive drum. The light beam scanning device of this type employs a method of simultaneously scanning a plurality of laser beams, so as to adapt to higher printing speed and higher resolution. For an apparatus that forms an electrostatic latent image on the photosensitive drum using a plurality of laser beams, rotation adjustment of a laser device is performed during assembly of the apparatus, so as to adjust relative image forming positions of the plurality of laser beams in the direction rotation of the photosensitive drum (sub scanning direction). By performing the rotation adjustment of the laser device, it is possible to cause the intervals of pixels in the sub scanning direction obtained by developing the electrostatic latent image to match a resolution.
Image forming apparatuses of these days are demanded to output high-resolution images. To meet the needs, the laser device is subjected to rotation adjustment such that the intervals of image forming positions of the plurality of laser beams in the direction of rotation of the photosensitive drum match a resolution. When the plurality of laser beams expose positions on a photosensitive drum shifted in the direction of scanning the photosensitive drum (main scanning direction), shifts in the main scanning direction are produced between pixels formed by the laser beams. Therefore, the timing of emission of each of laser beams from the laser device is controlled so as to prevent shifts in the main scanning direction between the pixels formed by the laser beams from being produced due to shifts of the laser beams in the main scanning direction.
On the other hand, in the image forming apparatus that forms an electrostatic latent image on the photosensitive drum using a plurality of laser beams, deviation of the relative positional relationship between dots (pixels) formed by respective laser beams from a desired positional relationship produces a periodical image shift, which causes moiré and like harmful effects on an image. To overcome this problem, it is necessary to adjust the laser device during assembly of the apparatus with high accuracy. Japanese Patent Laid-Open Publication No. H09-11538 proposes to adjust differences in scanning length between a plurality of laser beams on a laser beam-by-laser beam basis and adjust writing start positions of the respective laser beams on a laser beam-by-laser beam basis.
However, the shift in exposure position in the main scanning direction between laser beams sometimes differs depending on each position in the main scanning direction. In such a case, the adjustment of the entire scanning length and the adjustment of the writing start position alone are not enough for the adjustment of the dot position shift. In the following, a description will be given of a case where the amount of exposure position shift between laser beams varies with each scanning position.
In a light beam scanning device that converges laser beams onto a photosensitive drum via a lens, field curvature, which is a phenomenon in which a focus position varies with each position on a scanning surface, is caused depending on a molded state of the lens, and the field curvature also causes an exposure position shift.
FIGS. 9A to 9C show the relationship between a focus position shift and a shift in the main scanning direction in position of each of pixels formed by a plurality of laser beams. In this example, for simplicity of explanation, a description is given of an image forming apparatus that exposes a photosensitive drum by four laser beams 1, 2, 3, and 4. FIGS. 9A to 9C illustrates four pixels which are obtained by developing electrostatic latent images formed on the photosensitive drum by exposing the same sequentially from an upper side as viewed in FIGS. 9A to 9C using the respective laser beams 1, 2, 3, and 4. FIG. 9D shows the relationship between the focus position of a laser beam in the main scanning direction and the position of the surface of the photosensitive drum.
As shown in FIG. 9D, the focus position of the laser beam varies with the position on the surface of the photosensitive drum in the main scanning direction. When the laser beams are in focus on the photosensitive drum surface, the four pixels formed by developing electrostatic latent images formed on the photosensitive drum by exposing the same using the respective laser beams that form image thereon are at the same position in the main scanning direction (left-right direction, as viewed in FIG. 9A). The state illustrated in FIG. 9A is an ideal state in which the pixels formed by the laser beams are not shifted in the main scanning direction. However, when the focus positions of the laser beams are forward of (toward a near side with respect to) the drum surface, the lengths of optical paths of the plurality of laser beams are changed, and hence the relative positional relationship between the exposure positions of the laser beams deviates from a proper one. In this case, four pixels formed by developing electrostatic latent images formed on the photosensitive drum by exposing the same using the respective laser beams are shifted in the main scanning direction as shown in FIG. 9B. Similarly, when the focus positions of the laser beams are rearward of (toward a far side with respect to) the drum surface, four pixels formed by developing electrostatic latent images formed on the photosensitive drum by exposing the same by the respective laser beams are also shifted in the main scanning direction as shown in FIG. 9C.
In a scanning optical system using a polygon mirror, the focus position with respect to the scanning surface is made substantially constant e.g. by an f-θ lens. However, there is a limit to adjustment of the focus position by the f-θ lens, and hence the above-mentioned field curvature occurs, i.e. the focus position varies with each position on the scanning surface. As described above, in the case of the image forming apparatus that performs image formation using a plurality of laser beams, when the focus positions of the laser beams are shifted with respect to the position of the exposure surface of the photosensitive drum, the positions of pixels are shifted.